This invention relates to mullite and more particularly to the preparation of mullite whiskers.
Various methods have been used to produce mullite whiskers. U.S. Pat. No. 3,104,943, entitled "Spinnable Mullite Fibers and Their Preparation," which issued to Kenneth L. Berryoon Sept. 24, 1963, discloses a method whereby mullite fibers are crystallized from a vapor phase derived from heating sources of Si, Al, O.sub.2, and S in an atmosphere containing at least 1 percent H.sub.2 at 800.degree.-1200.degree. C.
U.S. Pat. No. 3,607,025 entitled "Silica-Deficient Mullite Fiber And A Process For Its Production," which issued to Howard W. Jacobson on Sept. 21, 1971, discloses a process whereby an alkali halide and AlCl.sub.3 are contacted with a silica source in the presence of an oxidizing gas at 1000.degree.-1350.degree. C. The fibers produced, however, are not stoichiometric mullite (72.8% Al.sub.2 O.sub.3 and 28.2% SiO.sub.2) by composition but do exhibit the x-ray pattern of compositional mullite. The fibers are silica deficient containing 82-99 percent by weight Al.sub.2 O.sub.3 and the remainder SiO.sub.2. Also in the process, the fibers must be leached out from a solidified melt with water or dilute acid solution.
U.S. Pat. No. 3,073,770, entitled "Mullite Synthesis," which issued to William R. Sinclair and John C. Williams on Jan. 15, 1963, discloses a process whereby a reaction sputtered film of Al.sub.2 O.sub.3 and SiO.sub.2 is heated to at least 1000.degree. C. to form mullite whiskers.
Bella Locsei in "Mullite Formation in the Aluminum Fluoride-Silica System (AlF.sub.3 -SiO.sub.2)," Nature, No. 4779, June 3, 1961, discloses that mullite can be formed from reagent grade aluminum fluoride (AlF.sub.3) with or without structural water and silica gel or silica glass by heat treatment in an oxidizing atmosphere.
Our (Inna G. Talmy and Deborah A. Haught) copending application entitled, "Mullite Whisker Preparation," U.S. patent application Ser. No. 07/070,757, filed on July 6, 1987, discloses a method of preparing mullite whiskers from AlF.sub.3 and SiO.sub.2 powders. First, an intimate, anhydrous mixture of AlF.sub.3 and SiO.sub.2 powders is heated in an anhydrous SiF.sub.4 atmosphere at a temperature of from about 700.degree. C. to 950.degree. C. to form barlike topaz crystals. Next, the barlike topaz is heated in an anhydrous SiF.sub.4 atmosphere at a temperature of from about 1150.degree. C. to about 1700.degree. C. to produce the mullite whiskers. The AlF.sub.3 starting material is expensive and it would be desirable to replace it with a less expensive material as a source of Al.sub.2 O.sub.3 in mullite.